If it weren't for Facebook CEO Mark Zuckerberg, Travis Kilgore wouldn't have met his wife, Heather.

He wouldn't have started an online correspondence with a man in Tehran, Iran, and gained insight into civil oppression in that country.

Worst of all, he wouldn't have been able to reconnect with his first-grade teacher, Anita Horton, to tell her how she helped convince him to embrace equality and abhor prejudice.

"I would kiss Zuckerberg on the mouth for allowing me the opportunity to tell her that," Kilgore, 39, says of the exchange, one of an estimated 70 billion messages sent every week among users of the world's largest social network.

Like many of its 1.26 billion members, Kilgore's life was irrevocably changed 10 years ago when Zuckerberg and a handful of classmates founded Facebook in a Harvard dormitory on Feb. 4, 2004.

A Rossville-based musician, Kilgore describes himself as a Facebook "junkie," but when he created his profile in 2008, he was skeptical about the site's ability to offer features that weren't already covered by other services, such as LiveJournal and MySpace. In the years since, however, the site became his and many other people's primary means of connecting online.

"It is the best and the worst of the Internet," Kilgore says. "Everywhere the Internet goes, Facebook is sure to follow.

"There are trolls and jerks on Facebook; there's social justice; there's meanness and mercy. Basically, the entirety of the human condition is on Facebook."

MORE THAN MYSPACE

Over the last decade, Facebook's rate of growth has more closely resembled that of bacterial colonies than the average Internet startup.

Zuckerberg and his co-founders initially limited the site's membership to Harvard students, opening the doors a few months later to other Ivy League schools, including Stanford, Columbia and Yale universities. According to the company's official timeline, it attracted its first million users within 10 months.

As Facebook became available to a wider potential audience, its ranks ballooned. By the end of 2005, students at more than 800 public schools could create accounts, and Facebook membership increased six-fold.

To Facebook's first adopters, the early days were defined, in part, by a competition with rival online network MySpace. At the time, it was a case of David and Goliath or ancient Sparta and Persia, but despite its lack of options for customizing its profiles, Facebook's sparse design and tools for communication were attractive to those who wanted something different, says Megan O'Dea, of Chattanooga.

O'Dea, 27, worked for the past three years as a social media specialist at Riverworks Marketing. She joined Facebook in 2005, when the site opened up membership to students at Appalachian State University, where she was a freshman. In retrospect, she says, it's not hard to see why Facebook was so appealing.

"Facebook had a cleaner interface than Myspace and a fresh reputation," O'Dea says. "Myspace had a reputation at that point for being ... very high school, very juvenile.

"At least for my age group, when Facebook came out, it kind of felt like graduating from high school to college. It was like, 'Oh, here is a social network for people my age. Now I can leave high school behind.'"

Making sure that members didn't feel a similar sense of graduating beyond relevance to Facebook fueled a decision by company executives in 2006 to no longer require that new profiles be tied to university email accounts.

At the time, Facebook's network of 6 million users was estimated to be about one-fifth that of MySpace, which had unrestricted public membership. Some saw Facebook's exclusivity as its underlying strength, but in a September 2006 interview with The New York Times, Zuckerberg said Facebook couldn't afford to remain restricted.

"We have two years of alums already, and more than one-third of the people using the site are not in college any more," Zuckerberg said. "If we make it so other young people can use the site, it strengthens the experience for everybody."

The decision proved fateful. Within 18 months of opening membership to the public, Facebook closed the gap with MySpace. According to Internet analytics firm ComScore, the two networks were attracting the same number of unique visitors each month in April 2008. By the end of that year, Facebook had more than 100 million more users than MySpace, according to ComScore.

THE BIGGER THEY ARE ...

In 10 years, Facebook has become a digital juggernaut of staggering proportions.

If it were a country, its 1.26 billion members would make it second only to China in size. Its content numbers in the hundreds of millions of gigabytes, and its daily usage around the world is measured in billions of minutes.

Like Google, Facebook's name has even officially graduated from noun to verb: "to spend time using the social networking website Facebook" or "[to] contact someone via Facebook," according to the Oxford Dictionary.

Nevertheless, there are some signs that point to Facebook's open-door policy causing younger users to jump ship for rival services such as SnapChat, which are less popular among older users and thus less likely to be policed by their parents.

According to an October 2013 report by market analyst Piper Jaffray & Co., only 23 percent of teens now consider Facebook to be their most important social network, a decrease of about 20 percentage points from 2012. During its third-quarter earnings report, also in October, Facebook verified the trend.

"We did see a decrease in [teenage] daily users [during the quarter], especially younger teens," Facebook chief financial officer David Ebersam said during the Oct. 30 meeting, before adding, "This is of questionable significance."

Analysts suggested this crumbling at the edges bodes ill for the network, but Facebook's most recent news was more heartening. The company's fourth quarter earnings report, released last week, showed profits of $523 million, an increase of more than 700 percent over the same quarter in 2012.

O'Dea says Facebook shouldn't be concerned, even if teens are losing interest.

"Businesses have invested thousands and millions of dollars into renting ad space on Facebook; they'll be slow to let that go," she says. "They'll stay as long as there's a captive audience, and there is always going to be a captive audience on Facebook as long as Facebook grays and gets an older user base. That's where the money is."

A POST-FACEBOOK WORLD

Whatever its future may hold, Facebook stalwarts say it has made an indelible impact on social media and the Internet.

"It's kind of wild to think about how many people are on the service using it," says Strat Parrott, 31, a Chattanooga-based Internet marketing and brand development specialist. "When [people my age] started in the mid-2000s, we were the first few million people, and it really is just mind-blowing that there are people all over the world using it.

"Basically, there's a larger population on Facebook than the whole United States. That's just wild to think about."

Even though nothing remains static forever, least of all on the Internet, whatever service dethrones Facebook inevitably will be influenced by its existence. In some ways, Kilgore says, Facebook's first decade has helped to transform social networking from a niche activity for undergraduates to a vital tool of digital communication with global appeal.

"Something else will come along. It's the nature of things," he says. "But I do not think, barring an asteroid striking the earth or some other apocalypse, that we'll be without a social network again.

"It may not be Facebook, but there will always be something."

Contact Casey Phillips at cphillips@timesfree press.com or 423-757-6205. Follow him on Twitter at @PhillipsCTFP.